This invention relates to a new bacterial strain and to a fermentation process for producing ethanol utilizing the bacterial strain.
Presently there are a variety of fermentation processes for producing ethanol from different feedstock materials and having different organisms. Corn stover presently is an abundant source of cellulosic material which consists of the leaves and stalks of the plant remaining after the removal of the cob. In the United States, in 1979, 6.4.times.10.sup.9 bushels of corn were produced. The amount of corn stover associated with this corn starch production and at a theoretical yield utilizing 100% of the available carbohydrate in the corn stover, can represent a quantity of biomass capable of producing 14,000,000,000 gallons of ethanol; roughly 10% of the domestic gasoline consumption in 1979. Presently there are a number of problems to the realization of this potential. When a yeast such as Saccharomyces cerevisiae is used for ethanol production, only the hexose portion of the biomass (approximately only 55% of total biomass) can be fermented. In addition, these hexose sugar polymers must be hydrolyzed prior to fermentation. Although this has previously been accomplished by high temperature and chemical pretreatment, or more recently, by enzymatic hydrolysis following addition of cellulases obtained from the fungus Trichoderma reeseii, it is accompanied by the serious drawbacks of high cost as well as low yields of fermentable sugars. As an alternative to these multi-stage methods involving prehydrolysis of cellulose, it has been proposed to ferment insoluble cellulosics to ethanol with the thermophilic, cellulolytic, anaerobe, Clostridium thermocellum. Both the cellulose and hemicellulose fraction, making up about 40 and 30% respectively of the biomass substrate, such as corn stover, are hydrolyzed. The hexose is produced or metabolized predominantly to a mixture to ethanol, acetate and lactate.
However, this organism, like yeast, cannot ferment the pentoses cleaved from the hemicellulose. In order to increase the economic potential of this biomass feedstock, it is desirable to provide a means for metabolizing these sugars as well. It is known that the termophilic anaerobe Clostridium thermosaccharolyticum can metabolize hexose and pentose carbohydrates to produce ethanol. However, it is also known that this bacterium simultaneously produces lactic and acetic acids in amounts approximately equal to the ethanol produced. This is highly undesirable since the majority of the input feedstock carbohydrates utilized in the fermentation processes is metabolized to the undesirable co-products, acetic acid and lactic acid.
Accordingly, it would be highly desirable to provide a means for selectively forming ethanol from a cellulosic source while minimizing the production of the undesirable co-products, acetic and lactic acids. Furthermore, it would be desirable to provide a means for metabolism of pentoses to produce ethanol.